This invention relates to a fuel supply control method for an internal combustion engine having a turbocharger, and more particularly to a method of this kind which is adapted to perform a fail-safe function to counteract an abnormality in the functioning of intake passage pressure detecting means, so as to prevent local overheating in the interior of the engine cylinders caused by the malfunction of the above pressure detecting means.
A fuel supply control system adapted for use with an internal combustion engine, particularly a gasoline engine has been proposed e.g. by U.S. Pat. No. 3,483,851, which is adapted to determine the valve opening period of a fuel quantity metering or adjusting means for control of the fuel injection quantity, i.e. the air/fuel ratio of an air/fuel mixture being supplied to the engine, by first determining a basic value of the above valve opening period as a function of at least the intake pipe absolute pressure and then adding to and/or multiplying same by constants and/or coefficients being functions of engine rpm, intake air temperature, engine cooling water temperature, throttle valve opening, exhaust gas ingredient concentration (oxygen concentration), atmospheric pressure, etc., by electronic computing means.
In this proposed fuel supply control system, when a defect in the sensor body or a disconnection in the wiring occurs in the intake passage pressure detecting means that detects the intake passage absolute pressure of the engine, it becomes impossible to continue not only the above-mentioned air/fuel ratio control in a proper manner, but also to continue the operation of the engine. A possible measure to cope with such an accident is to effect the control by determining the quantity of fuel to the engine by the use of a constant absolute pressure value PB.sub.0 set in advance, in lieu of the value of an absolute pressure (PB) signal outputted by the above detecting means, thereby somehow enabling continued operation of the engine.
On the other hand, in an engine equipped with a turbocharger, when the engine is being supercharged at a high supercharging rate, the air/fuel ratio of the air/fuel mixture being supplied to the engine is set to a rich ratio, in order to obtain a high engine output responsive to the supply of a large quantity of fuel to the engine, as well as to effect cooling of locally overheated areas in the engine cylinders. That is, when the engine is being supercharged at a high supercharging rate, the intake air pressure and the intake air temperature increase so that the combustion temperature increases in the engine cylinders, which can cause certain local areas of cylinders, e.g. the tip of the ignition plug, to easily get overheated, resulting in inconveniences such as premature ignition of the air/fuel mixture. Therefore, it is essentially necessary to effect cooling of the overheated engine cylinders by fuel contained in the air/fuel mixture supplied to the engine while the engine is operating in a high-rate supercharged condition, in order to achieve proper driveability and long life of the engine.
However, if during engine operation in a high-rate supercharged condition which essentially requires a large quantity of fuel supply to the engine, the aforementioned constant value PB.sub.0 of the intake passage absolute pressure is used in place of actual intake passage absolute pressure PB detected by the absolute pressure detecting means, when an abnormality occurs in the functioning of the above absolute pressure detecting means, the quantity of fuel supplied to the engine becomes insufficient, resulting in inadequate cooling of the locally overheated engine cylinder regions, thereby causing abnormal combustion in the engine cylinders such as premature combustion.